On The Pulsational Stability of Blue Supergiants

The brightest blue supergiants represent the most massive evolved stars known. They are believed to be in the core helium-burning phase of evolution. To discuss the question of their stability, model interiors of core helium-burning stars with hydrogen-burning shells have been constructed and tested for pulsational instability (15-1000 M0). All models are found to be extremely stable against nuclearenergized pulsations despite the high radiation pressure. Consequently, any star with mass less than the critically stable mass on the main sequence should evolve into the blue-supergiant region without disruption. By combining the theoretical calculations with the available observational data for the I.arge Magellanic Cloud, a self-consistent set of parameters may be obtained for the Large Cloud as follows: (1) the coincidence of the mass of the most luminous supergiants with the critical mass on the main sequence, (2) the occurrence of this mass at about 60 M0, (3) a hydrogen abundance of X = 075 in the young Cloud population, and (4) a distance modulus of (m M)o = 18.7. We further verify theoretically the remarkable observed constancy of brightness of the most luminous B supergiants and their stability against disruption or large amounts of mass loss; any other kind of variability is likely to be due to atmospheric phenomena or to binary motion. The mass of the brightest Cepheids in the Magellanic Clouds is about 15 M0. In our Galaxy, the most luminous supergiants known have inferred masses which do not significantly exceed the critical mass, nor is there any certain evidence for supermassiveness among the observed 0 stars.